where can I download a "text" addon?
Select the text and copy it, then paste it into a text editor and save it as something like Add_mesh_round_cube.py (make sure to give it the .py extension).  Now you can load it into Blender like any other addon.  Open the User Preferences (Ctrl+Alt+U) and go to the Addons tab, then hit Install From File and select the file from your computer.  Then click the little checkmark next to it to enable it.

Note that, as others have mentioned, the specific addon you are looking at is already bundled in Blender by default, so you can just find it in the list and enable it.